In optical transmission systems, the transmission capacity of an optical transmission system which already exists can be enhanced by transmitting the optical data signals in polarization multiplex. For the purpose of transmitting optical data signals in polarization multiplex, in each case two carrier signals are generated with the same wavelength in at least one transmission arrangement, each of them being modulated by a data signal. Here, the first and second modulated signals are polarized orthogonally to each other. The orthogonally polarized modulated signals are combined into one optical polarization-multiplexed signal. This optical polarization-multiplexed signal is injected into the optical transmission fiber and is transmitted along the optical transmission link to a receiving unit. At the receiving end, the two orthogonally polarized modulated signals are recovered from the polarization-multiplexed signal on the basis of their wavelength and polarization.
In this situation, the retrieval of the two orthogonally polarized modulated signals from the polarization-multiplexed signal represents one of the problems in the polarization-multiplexed transmission of optical data signals. For this purpose a feedback criterion must be determined, from the optical multiplex signal which is transmitted, for use in controlling a polarization control element arranged at the receiving end. With the help of this polarization control element, controlled by reference to the appropriate feedback criterion, and for example a downstream polarization splitter or a polarization filter, the two modulated signals which are polarized orthogonally to each other are separated.
Various feedback criteria are known for controlling the separation of the two orthogonally polarized signals at the receiving end. The publication “Optical polarization division multiplexing at 4GB/S” by Paul M. Hill et al., IEEE Photonics Technology Letters, Vol. 4 No. 5, May 1992, discloses the use of coherent techniques in combination with pilot tones for the purpose of reconstructing or separating, as applicable, polarization-multiplexed optical signals. In addition, the publication “Fast Automatic Polarization Control System”, Heismann and Whalen, IEEE Photonics Technology Letters, Vol. 4 No. 5, May 1992, discloses a separation of polarization-multiplexed optical signals by reference to a correlation signal generated from the clock pulse which is recovered together with the optical signals received. In addition, the German patent application 10147892.5 discloses a frequency shift method for separating polarization-multiplexed optical data signals at the receiving end, in which use is made at the transmitting end of two carrier signals which have a differential frequency and, for the purpose of separating the two data signals at the receiving end, the spectrum of the data signals transmitted at the differential frequency is analyzed for the purpose of controlling a polarization control element.
In F. Heismann et al., “Automatic polarization demultiplexer for polarization-multiplexed transmission systems”, Electronics Lett. (1993) Vol. 29, No. 22, pp 1965/6, a fully automatic polarization demultiplexer for an optical polarization-multiplexed transmission system is proposed. The demultiplexer consists of an electro-optical polarization converter and a simple fiber-optic polarization splitter. The polarization converter continuously converts any arbitrary and fluctuating polarization states at the end of the optical transmission link into a fixed polarization state, and they are then separated out spatially by the polarization splitter.
S. Bigo et al., “10.2 Tbit/s (256x42.7 Gbit/s PDM/WDM) transmission over 100 km TeraLight™ fiber with 1.28 bit/s/Hz spectral efficiency”, OFC 2001 Tech. Digest, Postconference Edition, pp. PD25-1-3, presents a transmission system with high spectral efficiency, incorporating both polarization multiplexing and also wavelength multiplexing. In this, the transmission capacity is increased by the fact that channels which are located in the C and L bands are mutually combined and are so arranged that they can be better isolated spectrally by means of vestigial sideband filtering in the receiver. Here, one of the two sidebands of the transmission signal is filtered out at the receiving end of the transmission system.
In addition to this, the publication by Mike Sieben et al., “Optical Single Sideband Transmission at 10 Gb/s Using Only Electrical Dispersion Compensation”, Journal of Lightwave Technology, Vol. 17, No. 10, October 1999 discloses a method for single-sideband transmission of optical signals, in which an optical single-sideband signal is generated at the transmitting end from a digital baseband signal with the help of at least one Mach-Zehnder modulator, using a Hilbert transformation. By the transmission of a single sideband, the of fiber chromatic dispersion is reduced, and the optical bandwidth is increased.